Electron capture and transport by heteropolyanions: Multi-functional electrolytes for biomass-based fuel cells

Keggin heteropolyanions (POMs) are evaluated for use as an alternative to H-2 in the storage and transfer of electrons from alcohols (ROH) to 0, in biomass-based electrochemical energy-conversion devices. Pt(0) (present as 10 wt.% Pt on C) is used to catalyze the oxidation of alcohols under mild conditions by alpha-H3PMo12O40 (H(3)1) and H5PV2Mo10O40 (H(5)2). During these reactions, the POMs efficiently capture electron equivalents as they are rapidly reduced by intermediates generated during the Pt(0)-catalyzed oxidation of alcohols. Potentiometric data indicate that this results in the two-electron reduction of 1, and in substantial reduction of 2. Kinetic data show that the rate of POM reduction is limited by the rate of ROH activation at Pt. Upon exposure to O-2, Pt(0) catalyses rapid oxidation of the reduced solutions of both 1 and 2 to their (nearly) fully oxidized forms. Possible effects of the POMs on the activity of the Pt catalyst were assessed by measuring rates of Pt(0)-catalyzed O-2-oxidations of EtOH. Here, as in the anaerobic reduction of POMs, activation of EtOH on Pt(0) is rate limiting. Having demonstrated this, Keggin heteropolyanions were added, and their effects on overall rates quantified. In all cases, inhibition was observed, and the effect was more pronounced for Keggin anions with more positive reduction potentials. While the reactions involved are complex, these observations suggest that under certain conditions POMs may inhibit the rate-limiting activation of EtOH by Pt(0). (c) 2006 Elsevier B.V. All rights reserved.